Related is a Python library for creating nested object models that can be serialized to and de-serialized from nested python dictionaries. When paired with other libraries (e.g. PyYAML), Related object models can be used to convert to and from nested data formats (e.g. JSON, YAML).

Example use cases for related object models include:

• Configuration file reading and writing
• REST API message response generation and request processing
• Object-Document Mapping for a document store (e.g. MongoDB, elasticsearch)
• Data import parsing or export generation

Requirements

• Python (2.7, 3.5, 3.6)

Installation

Install using pip...

pip install related

First Example

import related

@related.immutable
class Person(object):
    first_name = related.StringField()
    last_name = related.StringField()

@related.immutable
class RoleModels(object):
    scientists = related.SetField(Person)

people = [Person(first_name="Grace", last_name="Hopper"),
          Person(first_name="Katherine", last_name="Johnson"),
          Person(first_name="Katherine", last_name="Johnson")]

print(related.to_yaml(RoleModels(scientists=people)))

Yields:

scientists:
- first_name: Grace
  last_name: Hopper
- first_name: Katherine
  last_name: Johnson

Second Example

The below example is based off of this Docker Compose example. It shows how a YAML file can be loaded into an object model, tested, and then generated back into a string that matches the original YAML.

version: '2'
services:
  web:
    build: .
    ports:
    - 5000:5000
    volumes:
    - .:/code
  redis:
    image: redis

Below is the related object model that represents the above configuration. Notice how the name-based mapping of services (i.e. web, redis) are represented by the model.

import related


@related.immutable
class Service(object):
    name = related.StringField()
    image = related.StringField(required=False)
    build = related.StringField(required=False)
    ports = related.SequenceField(str, required=False)
    volumes = related.SequenceField(str, required=False)
    command = related.StringField(required=False)


@related.immutable
class Compose(object):
    version = related.StringField(required=False, default=None)
    services = related.MappingField(Service, "name", required=False)

The above yaml can then be loaded by using one of the convenience method and then round-tripped back to yaml to check that the format has been maintained. The related module uses OrderedDict objects in order to maintain sort order by default.

from os.path import join, dirname

from model import Compose
from related import to_yaml, from_yaml, to_model

YML_FILE = join(dirname(__file__), "docker-compose.yml")


def test_compose_from_yml():
    original_yaml = open(YML_FILE).read().strip()
    yml_dict = from_yaml(original_yaml)
    compose = to_model(Compose, yml_dict)

    assert compose.version == '2'
    assert compose.services['web'].ports == ["5000:5000"]
    assert compose.services['redis'].image == "redis"

    generated_yaml = to_yaml(compose,
                             suppress_empty_values=True,
                             suppress_map_key_values=True).strip()

    assert original_yaml == generated_yaml

More Examples

More examples can be found by reviewing the tests/ folder of this project. Below are links and descriptions of the tests provided so far.

Documentation

Below is a quick version of documentation until more time can be dedicated.

Overview

The attrs library is the underlying engine for related. As explained in this article by Glyph, attrs cleanly and cleverly eliminates a lot of the boilerplate required when creating Python classes without using inheritance. Some core functionality provided by attrs:

• Generated initializer method (__init__)
• Generated comparison methods (__eq__, __ne__, __lt__, __le__, __gt__, __ge__ )
• Human-readable representation method (__repr__)
• Attribute converter and validator framework

The related project is an opinionated layer built on top of the attrs library that provides the following:

• Value fields that handle both validation and conversion to and from basic data types like str, float, and bool.
• Nested fields that support relationships such as Child, Sequences, Mappings, and Sets of objects.
• to_dict function that converts nested object graphs to python dictionaries. Made customizable (without resorting to monkey-patching) by the singledispatch library.
• to_model function that instantiated classes used by the de-serialization process going from python dictionaries to the related model.
• Conversion helper functions (to_yaml, from_yaml, to_json, from_json) for easily going between related models and data formats.
• @mutable and @immutable for decorating classes as related models without the need for inheritance increasing maintainability and flexibility.

Class Decorators

See the decorators.py file to view the source code until proper documentation is generated.

Field Types

Adding your own field types is fairly straightforward due to the power of the underlying attrs project. See the fields.py file to see how the above are constructed.

Functions

See the functions.py file to view the source code until proper documentation is generated.

Credits/Prior Art

The related project has been heavily influenced by the following projects that might be worth looking at if related doesn't meet your needs.

• attrs - The engine that powers related functionality.
• Django ORM - Object-relational mapping for Django that inspired related's design.
• cattrs - Alternative take for handling nested-objects using attrs.
• addict and box - Python dictionary wrappers that do not require a model.
• Jackson - Java-based technology for serializing and de-serializing objects.

License

The MIT License (MIT) Copyright (c) 2017 Ian Maurer, Genomoncology LLC

0.7.1 (2018-10-13)

• Add URL to related pypi page [#28]
• Make singledispatch an optional dependency for < python 3.4. Thanks [GhostofGoes].
• URLField bug [#20]
• Deprecation warnings fixed for python 3.7 [#27]

0.6.2 (2018-02-12)

• Contribution [GabrielDav]: TimeField and DateTimeField fields.

0.6.1 (2018-01-31)

• Strict Mode [Issue #8] throws an exception when receiving an undefined key.

0.3 (2017-06-23)

• New type: ImmutableDict
• Add function on TypedMapping
• Bug fixes in from_yaml and from_json functions.

0.2 (2017-06-05)

• Allow None by default in Typed Collections.

0.1 (2017-05-24)

• Initial release.